As a vehicle taking into account environmental issues, attention is now focused on hybrid vehicles that use a motor and engine together for driving the wheel. Some hybrid vehicles take advantage of the differential action of the planetary gear mechanism to control the rotational speed by means of the electric motor or motor generator connected to the planetary gear mechanism to drive the internal combustion engine at the optimum operating point. Such vehicles are configured to cover the excessive or insufficient driving force and engine braking force through the electric motor or motor generator, and further carry out energy regeneration in a deceleration mode to improve the fuel efficiency of the internal combustion engine.
Japanese Patent Laying-Open No. 2005-237119 discloses the technique of preventing the transmission of excessive torque to the output member of a power split device in a hybrid vehicle of the above-described configuration by increasing the rotational speed of the motor generator, when the rotational speed of the output member becomes lower by at least a predetermined amount, to suppress generation of inertia torque from the engine.
However, in the case where periodic torque variation is applied to the drive shaft such as in the case of running on a bumpy road in which irregularities continue successively and periodically, the variation will be transmitted to the input shaft through the power split device from the engine towards the power split device. When the frequency of this torque variation approximates the unsprung resonance point of the main body, variation at the drive shaft will increase to cause greater variation at the input shaft. If the unsprung resonance point matches the torsional resonance point of the input shaft, excessive torque will be generated at the input shaft.
Thus, since the generated torque will be excessive and periodic in the case where the input variation towards the input shaft varies periodically and intermittently, control of the motor generator cannot keep up with the variation. Accordingly, vibration cannot be suppressed and will become noticeable.